1. Field of the Invention
The invention relates to an internal combustion engine system, a method of determining the occurrence of an air-fuel ratio imbalance state therein, and a vehicle.
2. Description of the Related Art
This kind of internal combustion engine system has been proposed that includes a multi-cylinder internal combustion engine, in which each cylinder is provided with a fuel injection valve, and an air-fuel ratio sensor disposed downstream of a combining portion, at which flows of exhaust gas from the cylinders of the internal combustion engine are combined, and that determines whether there is variation in air-fuel ratio between the cylinders of the internal combustion engine (see Japanese Patent Application Publication No. 2008-309065 (JP-A-2008-309065), for example). This system calculates the amount of exhaust gas for each of the cylinders based on a rotation speed and a load factor of the internal combustion engine, performs calculation of the amount of fuel at the combining portion, at which the flows of exhaust gas are combined, by dividing the amount of exhaust gas at the combining portion obtained from the calculated amount of exhaust gas of each cylinder by a detection value sent from the air-fuel ratio sensor, and estimates the amount of fuel for each of the cylinders based on the calculated amount of fuel at the combining portion with the use of an observer. Then, the air-fuel ratio for each of the cylinders is calculated by dividing the calculated amount of exhaust gas of each cylinder by the estimated amount of fuel of each of the cylinders and it is determined whether the amount of variation in air-fuel ratio between the cylinders is excessively large, based on the calculated air-fuel ratio of each of the cylinders.
In the above system, in order to determine the variation in air-fuel ratio between the cylinders of the internal combustion engine, various calculations are required, such as calculation of the amount of exhaust gas for each of the cylinders, calculation of the amount of fuel at the exhaust gas flow-combining portion, and estimation of the amount of fuel for each of the cylinders. In addition, it is also required to design the observer for observing the state of the amount of fuel of each of the cylinders based on the amount of fuel determined at the exhaust gas flow-combining portion in advance. In particular, when the observer is designed with the use of a simple model, a highly accurate output cannot be obtained and therefore, there is a case where it is difficult to properly design the observer. Thus, it is desirable to facilitate the determination of the variation in air-fuel ratio between the cylinders.